The control of mammalian cell proliferation by mitogens occurs largely during the G1 phase of the cell cycle. During this period, extracellular signals are transduced by cytoplasmic signaling cascades to the nuclear cell cycle clock, where a decision is made between cell cycle progression/proliferation and quiescence. Among the earliest responses to mitogenic signaling is activation of transcription factors such as c-Jun, a subunit of activator protein 1 (AP-1). Transcriptional activation of target genes by c-Jun and other immediate early transcription factors is thought to be essential for mitogen-induced progression through the cell cycle. A causal role has been attributed to c-Jun in promoting cell proliferation but the precise mechanism of c-Jun activation in this process needs further clarification. We initially identified a Jun-activation-domain binding protein (JAB1), which interacts with and potentiates transactivation by c-Jun and promotes cellular proliferation. Since then, a growing family of JAB1 transcriptional co-activators has been discovered and emerging evidence indicates that this JAB1 family plays a major role in co-integrating mitogenic signal transduction pathways involved in the control of proliferation. Yet the precise characterization of JAB1 has not been elucidated. Moreover, JAB1 protein levels are increased in transformed cells and the increase correlates with degree of tumor invasiveness. JAB1 has also been implicated in promoting cell proliferation by facilitating relocation of the cyclin-dependent kinase inhibitor (CDK) p27 from the nucleus to the cytoplasm, thereby accelerating its degradation by the ubiquitin/proteasome pathway. p27 plays an important role in the regulation of the G1 to S phase transition. Thus, an improved comprehension of the mechanisms regulating p27 expression and function could contribute to a better understanding of cell growth. Our preliminary results and recent findings strongly suggest that the Jabi gene plays a key role in the development of tumorigenesis. Our long-term goals are to define the physiological functions of JAB1 and to determine its role in the control of proliferation and tumor progression. We propose here to characterize the functions of the JAB1-c-Jun interaction implicated in the control of cell proliferation. We also hypothesize that JAB1 overcomes the inhibitory effect of p27 on cell cycling and facilitates progression to S phase. To further clarify the molecular mechanisms involved in JAB1-mediated downregulation of p27, we will identify the structural domain of JAB1 that interact with p27 to determine the effect of JAB1-mediated degradation of p27 on the proliferation of transfected cells. Finally, we will identify new targets or regulators of JABI by using a modified yeast two-hybrid system, the Ras recruitment system. Genes that are activated or factors that are regulated by JAB1 are probably critical components in the oncogenic pathway. Using JAB1 to re-establish a G1 checkpoint in tumor cells to block proliferation could represent a powerful therapeutic tool.